1. Field of the Invention
The present invention relates generally to a sprocket mechanism for cameras, and more particularly to a sprocket mechanism adapted for use with a newly developed film construction to ensure positioning of a single frame of the film at every advancing action.
2. Description of the Prior Art
FIG. 1 shows an existing film roll structure 10 which comprises a substantially hollow cylindrical magazine 11 and a spool 13 axially disposed in the center of the magazine 11. A film 14 is wound around the spool 13 and may be pulled via a notch 12 at a side of the magazine 11. The top and the bottom sides of the film 14 are provided with a multiplicity of sprocket holes or perforations 15. This kind of film rolls have been used for many years and are quite practical indeed.
With the development of modern technology, some manufacturers have recently developed a new kind of film rolls which are more performing and compact in size than conventional ones. Referring to FIG. 2, such a new film roll 20 is mainly characterized in that a top side of a magazine 21 is configured to have four different signs 22 and that a location plate 23 located at an inner wall of the top side of the magazine 21 will, following the action of the film roll 24, be situated at one of the signs 22 to represent the use status of the film roll 20. And besides, a film 25 of the film roll is provided with two juxtaposed sprocket holes 26 at regular space intervals (such as those illustrated in FIG. 2) for stressing the uniqueness of each individual frame. In other words, the exposure position of each frame is predetermined, unlike the conventional film rolls in which the exposure position of each frame is determined by the length of the leader pulled out when mounting the roll into the camera and by the length of the film wound upon film advancing, hence the lack of efficient film planning. The new film roll also provides an improvement for matching some structural parts of the camera to permit tight winding of the film 25, further reducing the size of a film compartment defined between the magazine 21 and the spool 24 for accommodating the film 25. In order to match the new kind of film rolls recently developed, novel improvement on the construction of the camera is therefore necessary.
With reference to FIG. 3 which shows a sprocket mechanism 30 and a roller mechanism 31 for assisting the advancing of films in conventional cameras, both the sprocket mechanism 30 and the roller mechanism 31 are driven by a motor via a plurality of gears 33 and planetary gears 34 to perform an advancing action on the film. However, it may be found that the sprocket mechanism 30 is proportionally provided at its circumferential rim with eight pawls 301 which may randomly engage with the series of sprocket holes of the film for transmission purposes. This arrangement, however, cannot and do not suit the new kind of film roll 20 with pre-determined film planning.